1. Field of the Invention
This invention relates to a self-timed method for compensating for track eccentricity or runout in a rotating recording disk. More particularly, it relates to a technique for entering compensation data onto the disk by the drive recording information to be stored onto the disk, which compensation data may be subsequently used, without the need for reading a separate clock track, to correct track runout during playback by the same or another drive unit.
2. Discussion of the Prior Art
In many data storage systems, particularly in the field of data processing, recording and information storage utilize a disk-type format. In many cases it is necessary to position a playback transducer or "read head" at a desired data track on the disk with great precision. This is because the information tracks on such disks are often in the form of concentric circles that may be only of the order of 1 milliinches wide and spaced with even lesser distance between tracks on the disk. If the transducer is improperly positioned, information recorded on an adjacent track may be sensed and thus lead to erroneous data recovery. The minimum track spacings are thus directly dependent upon the accuracy with which the transducer can be positioned relative to the disk.
When information is being retrieved from the disk there may be a lack of concentricity between the center of rotation of the disk and the circular data track previously recorded on that disk. This problem is of special concern in disk drives where removable media and interchange of data between drives is included as a design parameter. Differences in the mechanical construction between disk drive units may create such an eccentricity during playback on a disk drive other than the one used to create the disk. Even when the disk is to be be read by the same disk drive on which it was created, if the disk has been removed for one reason or another and then reinserted, it is sometimes quite difficult to return the disk to its original center of rotation. Consequently, the centers of the circles describing the information tracks no longer correspond to the center of rotation of the disk. Without compensation, the read head transducer may read information from adjacent tracks in an overlapping manner depending on the degree of eccentricity or runout, in which case the output from the transducer would be erroneous.
Several types of systems have been proposed to compensate for tracking error during playback. Examples of some recent developments are shown in U.S. Pat. Nos.: 4,135,217; 4,149,199; 4,157,577; 4,188,646; 4,286,296; and 4,371,960. One type of system utilizes a plurality of servo control tracks per surface to establish a continuous runout error correction signal around the circumference of the disk. Another system reads runout error in at least three segmental positions of rotation of the disk and computes a circle of errors passing through the runout error measured at the three reference positions. That circle is then stored and used to control the read head positioning during data recovery.
Other such systems employ a pair of transducers, one of which is dedicated to read servo reference information from one or more tracks which is used to control servo mechanism correcting the positioning of the playback transducer. Still other systems utilize combinations of the above, including having servo control information appear in a plurality of sectors around the disk on each track such that correction information is generated periodically during rotation of the disk. Each data track thus contains its own transducer position control information.
Each of these systems requires extensive data processing implementation to decode the servo reference information available and to then place that information in a form useful for controlling the position of the playback transducer. Moreover, generally these prior systems require the presence of a clock track to establish proper timing for the decoding of the servo reference information.